Enforcers (also called Space Policemen, skiny marines. ManMarines, IronManMarines) are Mantic equivalent to Spessh Mareeeens, more specifically to Ultramarines, though their models do look quite different, and their background, while following similar archtypes has began to develop very differently specially since deadzone provided us with more info about them.
Overall, enforcers are the GCPS elite army, but they are not really deployed en masse but rather used as a special ops squad, with a lot of focus on secrecy and lightning attacks, which would in many ways make them the polar opposite to certain space marine armies, while making them very similar to a certain non existent legion (to the point where every enforcer that is not a ranking officer is forbidden from revealing his face).
Militarywise enforcers are the closest thing you will get to reasonable marines, making use of sniper rifles, camouflage and engineering corps that dont believe technology is magical. Also despite being slimmer due to not having shoulderpads of power, they are actually a head taller than gw spesh mareens (some say primaris were created to compensate for that).
Overall enforcers are an interesting variant on the post-human archtype, though lorewise they are more similar to imperial stormtroopers than spesh mareens (since like stormtroopers they are enhanced humans that recieved military training instead of monastic knights utterly transformed into murderous machimes), some have even played dreadball before joining the ranks (thus making enforcers an army of former high school jocks).
After being originally released in restic, nowadays they are finally hard plastic and gorgeous, available mainly as normal troops (armed with laser guns or wristblades, which despite not being nearly as cool as chainswords might actually be more useful in a fight), peacekeepers (terminator equivalents that actually look like tartaros pattern terminators if they got a more square form, also like grim Buzz Lightyear) and enforcers on JETBIKES, thats right, this space marines know how to roll, which would also make them possible replacement for people who want to field a Saim-Hann host without selling kidneys since enforcers are 20 bucks for 4 vs Gw 35 for 3.
They also have Striders, which look like beefed up dreadnoughts (though admittedly missing some of the heraldry that makes dreadnoughts such a classic) and their Acusser flier that is basically bigger than a stormtalon, but smaller than a stormhawk and looks like a more advanced Valkyrie, so it could be either thing.
Overall enforcers are an interesting bunch of models, if you prefer some more realistic and serious approach to the super human archtype.
More comparison to Space Marines
Ok, so what makes the enforcers be more than just poor man's space marines? Well, first of all, while both serve a quasi-totalitarian regime, enforcers have much less autonomy than space marines, since enforcers are said to be under the direct rule of the council of seven, with little to no free will when it comes to deciding when and were to deploy.
Also, lorewise, while the space marines are organized into chapters, each a fairly individual self contained army, enforcers are instead considered a single fighting force and mostly using the same colour scheme, with the exception of some characters.
In essence, whereas space marines are a collection of knightly orders each mostly individual, enforcers are a single really big army, that mostly suppresses individual identity in favour of the army's uniformity.
Amazingly though, while the enforcers tend to supress individuality within their ranks, their soldiers, tend to have somewhat a story before joining the army. With many of them having even been dreadball players.
Another key difference between enforcers and space marines comes in the shape of mortality, enforcers are not only much more prone to dying than space marines, but also age, which means some have to retire, the dossiers says that when they do they go to a pleasure world (either that or they are sacrificed and uses as material, whatever sounds more comfortable).
In the end Space Marines are genetically enhanced humans that fight for the glory of the emperor, enforcers are soldiers that fight for the council of the 7 in order to gain money and a retirement package, quite a big difference in my opinion.